Batteries, such as those of the lithium/iodine type for example, sometimes referred to as solid state cells, make use of a cathode material comprised of iodine and an iodine containing charge transfer compound. Charge transfer compounds are sometimes referred to as complexes and sometimes as donor-acceptor compounds. The iodine in such a cathode reacts electrochemically with the lithium anode to provide a voltage output. This reaction causes a lithium iodide electrolyte to form in situ between the anode and cathode. The charge transfer donor typically used is a polyvinylpyridine, (PVP) such as poly-2-vinylpyridine (P2VP) or poly-4-vinylpyridine (P4VP). Additional amounts of free iodine ie., excess iodine which is not combined with the charge transfer donor are usually included as part of the cathode material to provide an iodine "reservoir" for the battery to draw on during discharge. The additional iodine increases the useful life of the battery.
Unfortunately, the polyvinylpyridine-iodine cathode compositions previously used, such as P2VP and/or P4VP based cathodes have exhibited a conductivity which degrades severely with the addition of relatively large amounts of iodine to the complex. Such amounts of iodine are hereinafter referred to as "excess iodine". Consequently, this degradation in the conductivity of the composition has effectively limited the amount of iodine which could heretofore be included in such cathodes without degrading the conductivity of the material below a useful level. As a result, the polyvinylpyridine cathode materials, which have been previously available, have not had as high an energy density as is theoretically possible from an iodine based system. This fact is particularly important for such applications as in implantable medical devices, ie., heart pacemakers for example.
It is a purpose of this invention to provide polyvinylpuridine-iodine cathodes, P2VP and/or P4VP based, having markedly higher conductivity than has been heretofore possible. As a result, the preparation of useful cathode materials having a much higher iodine content, such as materials having a final mole ratio of 20:1 or greater overall iodine to P2VP and/or P.sub.4 VP donor, is made possible. Consequently, higher energy densities and longer useful life are provided by the improved cathode materials prepared according to this invention.
Herein, "mole ratio" is defined in terms of the number of moles of iodine (I.sub.2) to the number of gram formula weights of vinylpyridine in the initial polymerdonor mixture. For example, a mixture which initially contains 508 grams of I.sub.2 and 10.5 grams of PVP would have a mole ratio (n) of 20:1 and will be designated as PVP.20I.sub.2. "Mole fraction" is defined in an analogous manner as n/n+1, according to the above nomenclature.
For descriptive purposes herein, the term "complex" refers to any single phase iodine and donor mixture. The term "cathode material" refers to a material composed of a "complex" and may include excess iodine, which may be present as a solid phase.